In January 1969, U.S. Pat. Nos. 3,421,692; 3,421,699 and 3,425,058 issued to Robert S. Babington and his co-inventors. These patents disclose a type of liquid atomization apparatus which is particularly useful in liquid fuel burners. The principle involved in the apparatus, now frequently referred to as the "Babington principle," is that of preparing a liquid for atomization by causing it to spread out in a free-flowing thin film over the exterior surface of a plenum having an exterior wall which defines the atomizer bulb and contains at least one aperture. When gas is introduced into the plenum, it escapes through the aperture and thereby creates a very uniform spray of small liquid particles. By varying the number of apertures, the configuration of the apertures, the shape and characteristics of the surface, the velocity and the amount of liquid supplied to the surface, and by controlling the gas pressure within the plenum, and quantity and quality of the resultant spray can be adjusted as desired to suit a particular burner application. Various arrangements of such atomization apparatus have been disclosed in other U.S. patents issued to the present applicant, namely U.S. Pat. Nos. 3,751,210; 3,864,326; 4,155,700; and 4,298,338. The disclosures of the patents mentioned in this paragraph are specifically incorporated by reference into this application.
So that liquid fuel burners and liquid atomizers constructed in accordance with the Babington principle will have the widest possible range of applications, it has been found desirable to provide the maximum possible variation in the volumetric flow rate of the atomized fuel or other liquid between the lowest and the highest flow rates required. For example, flow rates as low as 0.3785 liter (0.1 gallon) per hour may be required for some applications and as high as 3.785 liters (1.0 gallon) per hour may be required for others.
Once the particular geometry for a given atomization apparatus has been selected, however, changes in the flow rate of the atomized liquid must be made primarily by adjusting the flow rate of liquid onto the atomizer bulb. For the lowest flow rate desired, the liquid film thickness at the aperture preferably would be the thinnest achievable while still maintaining a continuous film over the exterior surface of the atomizer bulb. On the other hand, to provide higher flow rates of the atomized liquid, it is necessary to increase the thickness of the film at the aperture without increasing it so much that undesirably large liquid particles are formed. In prior art apparatuses, a single liquid feed tube has been positioned above each atomizer bulb so that a variable flow rate of atomized liquid from about 0.757 to 2.27 liters (0.2 to 0.6 gallons) per hour has been achievable.
While this type of prior art apparatus has been demonstrated to be a very efficient means for providing a spray of fuel for applications such as oil burners, erratic behavior occasionally has been observed during startup and particularly when the flow of liquid fuel over the atomizer bulb and the pressurized gas through the aperture are started simultaneously. Occasionally, an apparatus which had been functioning as desired for some time and then shut down for a period has been found to produce a stuttering, spluttering spray when fuel and air flow are started again after even a brief shutdown.
Continued research has shown that this erratic behavior can be due to the presence of air which becomes trapped in the feed tube to the atomizer bulb during shutdown, or to air entrained or dissolved in the fuel leaving the fuel pump, or to some combination of the two. Instability during startup can also be the result of surface tension and viscosity effects as the surface of the atomizer bulb is wetted during each startup procedure. As a result of such conditions, the flow of fuel leaving the feed tube may be somewhat irregular for a transient period during startup. During this transient period the surface of the atomizer bulb may not become completely covered with a thin film of fuel for as long as two or three seconds after the flows of fuel and air commence. During this time the quality of the spray of fuel is rather poor which can lead to difficulties in starting combustion, carry-over of raw fuel into the flame tube and other undesirable effects.
In some liquid fuel burners embodying the Babington principle, the flow rate of atomized fuel has been found to decrease somewhat as the temperature of the fuel increases during operation, apparently due to increased leakage in the pump and perhaps to changes in fuel properties as a function of temperature. In certain applications, however, it is considered desirable that the flow rate of atomized fuel leaving the aperture of the atomizer bulb should remain relatively constant as the temperature of the fuel varies, a mode of operation which has been difficult to produce with prior art burners.